(1) Field of the Invention
The present invention relates to water treatment and particularly to processes for converting sea, brackish, waste or otherwise impure water into water suitable for drinking and general usage while refining the concentrates and sediments resulting from the treatment. More specifically, this invention is directed to liquid treatment systems employing at least two membrane separation stages connected in concentrate sequence and especially to such systems wherein the concentrate produced by the last membrane separation stage is subjected to a thermal concentration step. Accordingly, the general objects of the present invention are to provide novel and improved methods and systems of such character.
(2) Description of the Prior Art
The use of membrane separation processes, such as reverse osmosis and electrodialysis, for the desalination of fresh water, brackish water, sea water and waste water is known in the art. Such processes have been found to be particularly well suited for use in the desalination of low salt content brackish water. In the prior art the achieveable product yield of a membrane separation process has been found to be limited by the materials contained in the water to be treated which, during the concentration process, exceed their solubility limit and thus precipitate out of solution. In other words, the achieveable product yield is not limited by the osmotic pressure of the concentrate. Accordingly, a problem known in the art as "membrane scaling", resulting from the precipitated deposits which form on the membrane, arises and these deposits have a negative influence on both the permeate flow and the salt retention characteristics of the membranes. In order to reduce "membrane scaling" in a desalination process, it is common practice to pretreat the "raw" water prior to its delivery to the desalination unit. Thus, by way of example, if carbonate hardness is the limiting factor in the formation of precipitates, acid will be added to the raw water and the carbonate hardness will be converted to non-carbonate hardness. Similarly, if there is a risk of calcium sulfate precipitation at the chosen concentration or permeate yield, it is necessary to reduce the calcium content of the "raw" water either by means of an ion exchange process or by causing a sufficient degree of chemical precipitation to permit obtaining the desired product yield. A further approach to preventing or minimizing "membrane scaling" is to delay precipitation by stabilization of the scale-causing compounds thereby achieving an improvement, albeit a very limited improvement, in the degree of concentration which can be achieved.
It is also to be noted that prior desalination processes employing membrane separation have typically called for the concentrate, which was produced by a single membrane separation unit, to merely be discarded. However, with very unfavorable "raw" water conditions, it becomes necessary to conduct the entire input quantity of "raw" water through an ion exchanger or a chemical precipitation reactor. If the concentrate produced by a single-stage membrane separation unit is discarded, dependent upon the product yield obtainable in such unit, a quantity of "raw" water which amounts to up to three (3) times the obtainable product yield must be treated by precipitation or ion exchange.
If an anion exchange step is performed, a weakly basic anion exchanger in chloride form is added to the "raw" water. This anion exchanger absorbs sulfate and releases chloride into the solution. While such an anion exchanger can be regenerated with the brine from a subsequent desalination step, it is necessary, either in the charging cycle or during regeneration, to add acid in order to obtain a sufficiently high sulfate capacity of the anion exchange resin. Further, and in any event, the useable volume capacity of a weekly basic anion exchange resin is only 1.2-1.6 val/l.sub.A. Capacities in the range of 2-2.5 val/l.sub.A may be achieved through the use of a weak acid cation exchanger which reduces the concentration of alkaline earth ions to a desired minimal value. However, the cation exchanger resin must be regenerated with hydrochloric acid and caustic soda. Regeneration with a sufficiently high quantity of acid is especially important in order to obtain the smallest possible calcium concentration in the softened water being produced. The use of a weak acid cation exchanger has been impeded because the neutral salts from the brine of a desalinizer can not be used directly for regeneration of the resin.
The high pressure pumps necessary to drive a reverse osmosis device are often driven by diesel engines. This is particularly true in oil producing countries where inexpensive fuel is available. The thermal energy produced in the burning of the fuel is converted, with about forty-one (41%) percent efficiency, into mechanical energy which can be applied to the desalination process. Of the remaining energy, approximately twenty-eight (28%) percent is used in the cooling of the desalination apparatus with water or air, approximately twenty-four (24%) percent is lost in the exhaust and the remainder is lost through other causes. Thus, prior reverse osmosis desalination techniques have, taking into account the losses, been characterized by inefficiency.
It is also noteworthy that the concentrates produced by the membrane separation unit of prior desalination systems have either been discharged into nearby surface waters, such as rivers, lakes or the ocean, or permitted to seep into the ground. These concentrates have a very high salt-content and thus their discharge into the environment presents the danger of salination of the surface waters or ground water. Accordingly, to the extent not already required, the controlled removal of such high salt content solutions will become mandatory in the near future. Thus, considering the operation of a desalination installation, the amount of concentrate produced will have to be substantially reduced in order to minimize the cost of transportation of the high salt content liquid waste to an appropriate disposal site or all of the salts in the concentrate will have to be converted to solid form.
It is noted that membrane separation processes, and especially reverse osmosis, are more energy efficient than alternative processes such as evaporation in, for example, the desalination of sea water. In a desalination process, depending on the salt content of the "raw" water, the osmotic pressure of the concentrate may reach a level of 60 bar. At present, the maximum operating pressure of a reverse osmosis system is 70 bar and, with a high salinity solution being treated, the maximum product yield will be forty (40%) percent. Concentrates produced in the desalination of waste water and brackish water using membrane systems with high product yields show salt content similar to that resulting from the desalination of sea water. The relatively large quantity of water discharge inherent in the above-discussed operating conditions is, accordingly, a significant limitation on the use of membrane technology. Thus, additional processes stages are necessary to further treat the concentrate so that the systems can be economically operated beyond the salinity limits of the reverse osmosis process. In an evaporation process the limiting factor on product yield is not the salinity of the concentrate or its osmotic pressure, but the risk of forming deposits on the heat exchanger surfaces as a result of precipitation of materials during concentration. This is a chemical problem which can be solved by suitable pretreatment thus enabling the preparation of high salinity solutions with evaporation systems.
The permeate obtained with prior membrane desalination installations is normally not of drinking water quality. Thus, the permeate will typically have an excess of sodium salts and carbonate deficiency. Also, the total salinity of the permeate is often too low. In order to increase the "hardness" of the permeate, it is often dosed with carbon dioxide and/or passed through a deacidifying filter including dolomite material or lime water is added to the permeate stream. The addition of carbon dioxide results in the alkaline calcium compounds being converted to calcium carbonates. In many cases the carbon dioxide is extracted from the air or produced by burning a fuel, this being particularly true in the less industrialized countries. This, of course, further increases the cost of the water treatment apparatus and system operation.